fix(utils): make all failing tests pass

[stooit]

Summary

Fixes all 16 failing tests across 5 source files in the utility library. Test suite now reports 60 pass, 0 fail. No test files were modified and no dependencies were added.

Changes

• src/calculator.ts — divide throws on division by zero (including -0) instead of silently returning Infinity/NaN.
• src/string-utils.ts — implemented truncate (word-boundary truncation with "..." counting toward maxLength; returns unchanged when within limit); fixed wordCount to split on /\s+/ so runs of whitespace no longer inflate the count.
• src/task-manager.ts — implemented remove (returns true/false), update (returns false for unknown id; partial-update safe via !== undefined checks), and sortBy (priority high>medium>low, status ordering, and createdAt oldest-first).
• src/date-utils.ts — formatRelative off-by-one fixed: round instead of floor so 36 hours reads as "2 days ago"; also corrects the future-dated sign case.
• src/validator.ts — isEmail now accepts long TLDs (e.g. .museum) and multi-level subdomains while still rejecting empty labels; isUrl now accepts URLs with a port (e.g. http://localhost:3000).

Verification

• bun test → 60 pass, 0 fail
• bunx tsc --noEmit → clean

Review notes / assumptions

A review flagged two out-of-scope edge cases in truncate that no test covers and that don't affect the documented contract for sane inputs, left as-is per the "fix only what the tests require" constraint:

• Negative maxLength returns "" (undefined behaviour).
• A string whose only whitespace is a leading space falls back to a mid-word cut.

isEmail remains a regex approximation (not RFC 5322) — suitable for form-level validation, not as a sole trust gate for delivery/identity.